Oka, on Hirudinea, [399] f.;

on Polyzoa, [500]

Olfactory pits, in Polyclads, [26];

in Triclads, [36]

Oligocelis, [42]

Oligochaeta, [241], [347] f.;

external characters, [348];

body-wall, [349];

chaetae, [350], [351];

branchiae, [352];